Authors: Xu Tang, Qin Ailing, Li Yizhuang
Objective
To establish a conventional intraocular lens (IOL) calculation formula which is applicable to eyeballs with abnormal data and laser in situ keratomileusis (LASIK) postoperative eyes.
Methods
A case-series study was adopted.According to the classical optical theory, a normal IOL implanted eye has the following characteristics: when light is refracted by cornea and arrives on the IOL plane, the value of refractive power (F1) + IOL refractive power (F2) = the value of refractive power which is suitable for vitreous body depth (F3). Thereafter, a mathematical model was built on the basis of theory, experience, and regression analysis data after IOL implantation surgeries.Furthermore, based on the new LASIK postoperative cornea curvature modified formula, the two kinds of IOL calculation programs of conventional and LASIK postoperative eyes were established.The test data was collected from 644 patients who had undergone the cataract extractions and IOL implantation surgeries (600 physiological cornea eyes, 7 radial keratotomy [RK] eyes and 37 LASIK postoperative eyes) at the Affiliated Drum Tower Hospital of Nanjing University Medical School.Through the analysis of these data, the new formulas were examined.
Results
With IOL refractive power of 607 eyes (including 7 RK postoperative eyes), the average error of XLQ formula (the tentative name of the established formula in this study)was 0.1 D, and the 95% limits of agreement range was -1.1 to + 1.2 D. The error range of IOL refractive power predicted by XLQ, SRK-T and Haigis formulas was -2.21 to + 2.25 D, -5.10 to + 5.63 D and -3.00 to + 3.18 D, respectively, the absolute average error of IOL refractive power predicted by the three formulas was (0.43±0.28), (0.74±0.53) and (0.79±0.49)D, respectively.Compared with SRK-T and Haigis formulas, the average error of IOL refractive power predicted by XLQ formula was Lower, with significant differences between them (both at P=0.000). The error value of IOL refractive power predicted by XLQ formula had no statistical correlations with axial length (AL), keratometry (K) and A constant respectively (all at P>0.05), while the error value predicted by SRK-T and Haigis formulas had statistical correlations with AL, K and A constant, respectively (all at P<0.05). Thirty-seven patients who had conducted LASIK for myopia (and whose IOL refractive power value were predicted by XLQ formula) had been undergone the postoperative examination.Comparing the predicted and actual value, the error range of IOL refractive power was -0.52 to + 1.18 D, and the absolute average error was (0.49±0.26)D.
Conclusions
The conventional mode of the XLQ formula established in this study can be used in the cases with broad values of axial length, corneal curvature and A constant, as well as various types of physiological cornea and RK postoperative eyes; the dedicated mode is suitable for LASIK postoperative eyes of myopia.